Parallel data fetching in NextJS - Performance & Optimization

Performance: Parallel data fetching

HIGH IMPACT

This affects page load speed by reducing the time waiting for multiple data sources to respond, improving the Largest Contentful Paint (LCP).

Fetching multiple API endpoints before rendering a page

NextJS

const [data1, data2, data3] = await Promise.all([
  fetch('/api/data1').then(res => res.json()),
  fetch('/api/data2').then(res => res.json()),
  fetch('/api/data3').then(res => res.json())
]);

All fetches run at the same time, reducing total wait to the slowest fetch only.

📈 Performance GainReduces blocking time by up to 3x; improves LCP and user experience.

Fetching multiple API endpoints before rendering a page

NextJS

const data1 = await fetch('/api/data1').then(res => res.json());
const data2 = await fetch('/api/data2').then(res => res.json());
const data3 = await fetch('/api/data3').then(res => res.json());

Each fetch waits for the previous one to finish, causing sequential delays.

📉 Performance CostBlocks rendering for sum of all fetch times; increases LCP significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sequential fetches	Minimal	1 (after all data)	High (delayed paint)	[X] Bad
Parallel fetches with Promise.all	Minimal	1 (after all data)	Low (faster paint)	[OK] Good

Rendering Pipeline

Parallel data fetching reduces the time spent in the data retrieval phase before rendering, allowing the browser to start layout and paint sooner.

→Data Fetching

→Rendering

→Paint

⚠️ BottleneckData Fetching (waiting for API responses)

Core Web Vital Affected

LCP

This affects page load speed by reducing the time waiting for multiple data sources to respond, improving the Largest Contentful Paint (LCP).

Optimization Tips

1Always fetch independent data sources in parallel to reduce total wait time.

2Use Promise.all to run multiple fetches concurrently in Next.js.

3Avoid sequential awaits for multiple data calls to improve page load speed.

Performance Quiz - 3 Questions

What is the main benefit of fetching data in parallel in Next.js?

A It reduces total waiting time for all data to load.

B It increases the number of DOM nodes.

C It delays the first paint.

D It blocks JavaScript execution longer.

DevTools: Performance

How to check: Record a performance profile while loading the page; look for long gaps in the 'Network' section where fetches happen sequentially versus overlapping.

What to look for: Overlapping network requests indicate parallel fetching; shorter total blocking time improves LCP.

Practice

1. What is the main benefit of using Promise.all for data fetching in Next.js?

easy

A. It fetches multiple data sources at the same time, reducing total wait time.

B. It fetches data sequentially to avoid race conditions.

C. It caches data automatically for offline use.

D. It delays fetching until the user clicks a button.

Solution

1. Step 1: Understand Promise.all behavior

   Promise.all runs multiple promises in parallel and waits for all to finish.

2. Step 2: Relate to data fetching speed

   Fetching multiple data sources at once reduces total waiting time compared to sequential fetching.

3. Final Answer:

   It fetches multiple data sources at the same time, reducing total wait time. -> Option A

4. Quick Check:

   Parallel fetching = faster load [OK]

Hint: Remember: parallel means 'at the same time' [OK]

Common Mistakes:

• Thinking Promise.all fetches sequentially
• Confusing caching with parallel fetching
• Assuming it delays fetching until user action

2. Which of the following is the correct syntax to fetch two APIs in parallel using Promise.all in Next.js?

easy

A. const [res1, res2] = await Promise.all(fetch(url1), fetch(url2));

B. const [res1, res2] = await Promise.all([fetch(url1), fetch(url2)]);

C. const res1 = await fetch(url1); const res2 = await fetch(url2);

D. const res1 = fetch(url1).then(fetch(url2));

Solution

1. Step 1: Check Promise.all argument format

   Promise.all expects an array of promises, so the fetch calls must be inside an array.

2. Step 2: Identify correct syntax

   const [res1, res2] = await Promise.all([fetch(url1), fetch(url2)]); correctly uses an array: Promise.all([fetch(url1), fetch(url2)]). const [res1, res2] = await Promise.all(fetch(url1), fetch(url2)); misses the array brackets.

3. Final Answer:

   const [res1, res2] = await Promise.all([fetch(url1), fetch(url2)]); -> Option B

4. Quick Check:

   Array of promises = correct syntax [OK]

Hint: Always wrap promises in an array for Promise.all [OK]

Common Mistakes:

• Forgetting the array brackets around promises
• Using sequential awaits instead of Promise.all
• Chaining fetch calls incorrectly

3. Given this Next.js code snippet, what will be logged to the console?

async function fetchData() {
  const [userRes, postsRes] = await Promise.all([
    fetch('https://api.example.com/user'),
    fetch('https://api.example.com/posts')
  ]);
  console.log(userRes.status, postsRes.status);
}
fetchData();

medium

A. 200 200

B. undefined undefined

C. Promise Promise

D. Error: fetch is not defined

Solution

1. Step 1: Understand what fetch returns

   Each fetch returns a Response object with a status property indicating HTTP status.

2. Step 2: Analyze Promise.all result

   Promise.all waits for both fetches and returns an array of Response objects. Logging userRes.status and postsRes.status prints their HTTP status codes.

3. Final Answer:

   200 200 -> Option A

4. Quick Check:

   Response.status = 200 if successful [OK]

Hint: fetch returns Response objects with status property [OK]

Common Mistakes:

• Expecting fetch to return JSON directly
• Logging promises instead of awaited results
• Confusing undefined with Response properties

4. What is wrong with this Next.js code that tries to fetch two APIs in parallel?

async function getData() {
  const res1 = fetch('https://api.example.com/data1');
  const res2 = fetch('https://api.example.com/data2');
  const data1 = await res1.json();
  const data2 = await res2.json();
  return { data1, data2 };
}

medium

A. Nothing is wrong; this code fetches data correctly.

B. The fetch calls are not awaited in parallel, causing sequential fetching.

C. The function should use Promise.all to fetch in parallel.

D. You cannot call json() on a promise; you must await the fetch first.

Solution

1. Step 1: Identify the type of res1 and res2

   Both res1 and res2 are promises from fetch, not Response objects yet.

2. Step 2: Understand json() usage

   You must await the fetch promise to get the Response object before calling json(). Calling json() directly on a promise causes an error.

3. Final Answer:

   You cannot call json() on a promise; you must await the fetch first. -> Option D

4. Quick Check:

   Await fetch before json() [OK]

Hint: Always await fetch before calling json() [OK]

Common Mistakes:

• Calling json() on a fetch promise without awaiting
• Assuming fetch returns JSON directly
• Not using Promise.all for parallelism

5. You want to fetch user info and user posts in parallel in Next.js, but only display posts if user info fetch succeeds. Which approach correctly handles this?

hard

A. Use Promise.all for both fetches and check user info response before rendering posts.

B. Fetch user info first, then fetch posts only if user info fetch succeeds.

C. Use Promise.allSettled to fetch both, then conditionally render posts if user info succeeded.

D. Fetch posts first, then fetch user info regardless of posts fetch result.

Solution

1. Step 1: Understand requirement for conditional rendering

   Posts should display only if user info fetch succeeds, so we need to know each fetch's success independently.

2. Step 2: Choose correct parallel fetching method

   Promise.all fails fast if any promise rejects, so it can't handle partial success. Promise.allSettled waits for all promises and reports each result, allowing conditional logic.

3. Step 3: Apply conditional rendering logic

   After Promise.allSettled, check user info status; if successful, render posts, else skip posts.

4. Final Answer:

   Use Promise.allSettled to fetch both, then conditionally render posts if user info succeeded. -> Option C

5. Quick Check:

   Conditional render needs allSettled [OK]

Hint: Use allSettled to handle partial success in parallel fetches [OK]

Common Mistakes:

• Using Promise.all which fails on first error
• Fetching posts before confirming user info success
• Not handling fetch failures gracefully